Triangulation is a well known method of determining position. The basic premise of triangulation is as follows: given that a triangle has three sides and three interior angles for a total of six items of measurement, when three of these items of measurement are known, the remaining three unknown items of measurement can be resolved.
Resection is also a well known method of determining position. Resection is a special application of the triangulation method of positioning. The basic premise of resection is as follows: given three points of known position and a point of unknown position; the coordinates of the unknown point can be calculated when the angles between lines linking the three points of known position to the point of unknown position are also known.
A thesis entitled "The Design and Development of a Mobile Robot Location System" by Theodore Scott Rappaport, published in Dec., 1984, discloses two methods for determining position. The first of Rappaport's methods (Method I) is based on the special application of triangulation called resection. The positioning system utilizes three light beacons located at known positions and a rotating detector head which rotates at a constant angular velocity. The rotating detector head is attached to a mobile robot located in a unknown position The angles between the lines of the three light beacons and the rotating detector head are determined through time measurements. A computer generates a time measurement each time a photodetector located on the horizontally rotating head detects a light beacon as the photodetector scans a horizontal plane. Time measurements are obtained as each beacon activates the photodetector.
Identifying the individual beacons, which is necessary in order to determine position, is accomplished by modulating one of the beacons differently than the other two beacons. A prototype mobile robot and positioning system was built of the above Method I system. Accuracies of .+-.100mm were obtained except in "zones of ambiguity", which were defined as the area where adjacent beacon angles were less than 30 degrees.
The second positioning method disclosed by Rappaport (Method II) is one in which rotating light beacons are fixed in known positions and a portable light-detector is attached to the mobile robot.
Both Method I and Method II as disclosed by Rappaport can work only as an x, y positioning system. Method I utilizes timing of a rotating mechanism on a mobile robot. This rotating mechanism requires a very stable platform which will accommodate the movement of the mobile robot and ensure a horizontal plane, thus limiting the practicality of the system for variable z-axis measurements. Method II suffers from limited z-axis control and did not expressly provide for the synchronization of the time between the fixed stations and the portable station.
U.S. Pat. No. 4,700,301 to Dyke discloses a method of automatically steering agricultural type vehicles on a predetermined course. In order to accomplish this, Dyke discloses a method of sensing the present position and direction of motion of the vehicle. The present position and direction are then compared to the predetermined course and the steering angle is adjusted to compensate for any differences.
More specifically, Dyke discloses installing a laser emitting a narrow beam of light and an optically aligned directional light detector on a rotating platform on the vehicle. Retroreflectors are placed at reference points around the field in which the vehicle is operating. When the laser light hits the retroreflectors, the reflected light is detected and the computer records counts corresponding to the times of the hits. The position of the vehicle can be determined from these counts.
Alternatively, Dyke discloses that the position of the vehicle can be determined using lasers rotating at a constant angular velocity located at two reference points, and an all directional light with a radio transmitter for transmitting a detect signal to the vehicle located at a third reference point. When the reflected light is detected, the computer records counts corresponding to the times of the hits. The position of the vehicle can be determined from these counts.
The method disclosed by Dyke provides only a method for determining position of a vehicle in an x-y plane. There is no provision for determining position in an axis perpendicular to the x-y plane, i.e., the z-axis Moreover, the method suffers from the disadvantage that it is limited to sites which are relatively planar.
U.S. Pat. No. 4,820,041 to Davidson et al. discloses a position sensing system for surveying and grading. The system includes two laser reference stations, each of which emits a laser beam that periodically sweeps across the area to be surveyed Each time a laser beam strikes the opposite reference station, a radio timing signal is broadcast by that reference station. Thus, each reference station must be connected by a line-of-sight to each other reference station so that all reference stations are on essentially the same elevation. The system also includes a portable sensing station having a laser beam receiver, a radio receiver and a programmed computer. The position of the sensing station in the x-y plane relative to the reference station is computed based on the relative timing of detection of the laser beams and the reception of the radio signals. The position of the sensing station in the z-direction, i.e., elevation, is determined according to the height at which one of the laser beams strikes the laser beam receiver.
The system disclosed by Davidson et al. suffers from several disadvantages. As Davidson et al. themselves acknowledge at column 24 of the patent, three such laser reference stations are required, as a practical matter, in order to improve the area of coverage and accuracy of the system. By using three such laser reference stations in an appropriate layout, the region of measurement ambiguity, which occurs in the area between two reference stations where the interior angles are very small and thus the intersection of the laser beams at the sensing station approaches a straight line, is eliminated. The incorporation of a third reference station is not described, and Davidson et al. does not disclose how this could be done in view of the need to have a line of sight between the reference stations in order to activate the timing datum. Furthermore, Davidson et al. does not disclose how the portable sensing station would identify the incoming laser beams from the individual reference stations, which is necessary in order to calculate position. This identification is a particular problem since a practical system will need a minimum of three laser reference stations, and this third laser beam will add significant identification complexity.
The four-sided linear array detector of the portable sensing station disclosed by Davidson et al. limits the range of vertical positioning to the length of the linear array, without some mechanical method of raising and lowering the four-sided linear array detector along the z-axis. In addition, the electronic circuitry required to manage the information from the four-sided linear array detector is complex. The vertical accuracy achievable in the Davidson et al. system with the four-sided linear array detector is a function of the height of the individual linear array detector elements, which is limited to a minimum height in order to provide sufficient photosensitive surface area to detect the laser beam. The horizontal (x, y) accuracy of the system disclosed by Davidson et al. is limited by the distance of the individual detectors of the four-sided linear array detector to the actual center of the portable sensing station, the position of which is being determined.
Moreover, the system disclosed by Davidson et al. requires alignment of each laser beam into a specific plane containing the laser and the reference photodetector corresponding to that laser. Thus, the measurement accuracy of the system disclosed by Davidson et al. depends, in a large part, on the distance of the portable sensing station from the fixed reference stations There is no disclosure of how the system could be adapted for use on a large non-planar site (i.e. typical construction site) where more than two laser reference stations would be needed to achieve a useful accuracy over the entire site.
Furthermore, the range in which all coordinates (x, y, z) of the portable sensing station disclosed by Davidson et al. can be determined is limited to the planes where both rotating laser beams will strike the four-sided linear array detector.